Its source is a Hebrew expression meaning "Pause."
The word selah occurs in the Book of Psalms in the Jewish scriptures 71 times. Scholars agree it was a marginal musical direction, possibly transcribed into the main text by mistake. However, they disagree about its meaning.
Some believe it was equivalent to a rest symbol in modern music (and modern literary allusions to the word reflect this interpretation), but others suggest it was the cue for a crash of cymbals to punctuate the musical incantation of the Psalm.